Petrography of Sandstone

Exercise #1 Just to get you started with the study of sandstone and other siliciclastic rocks there are 12 samples, some with hand samples that you should look over. Their features are characterized on the Key Sheet which you should print out, examine and bring to lab. Budget your time, expect to spend 5 minutes on each sample.

Sandstone Modal Analysis

We will use modal analysis as the primary means to observe sandstone and we will use the results of the modal analysis as the means to describe or classify sandstone. The modal analysis that you must perform has to be designed to meet these ends. In a past lab exercise you performed a modal analysis where you simply counted mineral types; here we must go for much more detail. The modal analysis must be done so that in the end we will be able to differentiate between all the various types of detrital grains, all the cements, matrix, authigenetic replacements, and the various pore types. This is not a simple task but it does follow the same procedures as the simple modal analysis that you did in last week's lab. During your initial evaluation of a thin section you should note the various grain types which from the framework of the sample and you should note what occupies the space between these grains. Grain types include detrital mineral grains (quartz, feldspars, micas, heavy minerals etc. which were derived from pre-existing rocks) and rock fragments (bits and pieces of pre-existing rock such as limestone, granite, slate etc). But you need more detail than that! More specifically for detrital quartz there is:

• monocrystalline quartz (single crystal lattice for entire grain), • polycrystalline quartz (two or more crystals of quartz within the grain), • chert (cryptocrystalline quartz forming the grain). • You could go further here but don't. If you take sandstone petrology in

the future you will!

Feldspars will be identified simply as monocrystalline:

• feldspars if you have no evidence to further refine your identification • plagioclase if you can see the twinning or have stained the sample. • orthoclase if you have stained the sample • microcline if you can see the tartan twin pattern • polycrystalline feldspar grain types can also occur.

Do your best to differentiate between each feldspar type but remember sometimes you must just say it is feldspar. Expect to find detrital grains of the various micas (muscovite, biotite, and chlorite) as well as grains of glauconite, phosphorite, zircon, and any of the other possible heavy minerals. You may also encounter as detrital grains as fragments of other rock types. These are called rock fragments. If you can identify the fragment as coming from a sedimentary rock then it is a sedimentary rock fragment (SRF). Which may further be identified as a:

• sandstone rock fragment, • limestone rock fragment, • shale rock fragment, • coal fragment, etc.

If the rock fragments were derived from a volcanic source then they are volcanic rock fragments (VRF); which would include basalt rock fragments. If the source were metamorphic then you would apply the term metamorphic rock fragment (MRF). These could be further subdivided into slate rock fragment, schist rock fragments etc.. Fragments derived from phaneritic igneous rock sources and some of the coarser metamorphic rock types like gneiss generally are identified as feldspar grains unless you are working with conglomerates then you could have igneous rock fragments (IRF) or more specifically granite rock fragments, diorite rock fragments, and also then the gneiss rock fragments. The whole trick is to identify the rock fragments down to the most detailed type possible. However, you will at times be forced to use the term rock fragment if identification is not possible. The closer that you can

get to identifying the rock fragment the closer you will be to knowing the source area of the sediment. Keep track of any fossils present; these are called bioclasts. They may be calcite, calcitized aragonite, silicified aragonite, silicified calcite, dolomitized calcite or aragonite, or carbon. Keep track of the different mineral types, don’t just call then bioclasts. If you can identify the type of fossil do so. What is between the grains? There could be

• open pore space, • cement, or • matrix.

Pore spaces should be described and tallied in any modal analysis. Pore types can be categorized into

• intergranular porosity (between grains), • intragranular porosity (pores within a grain), • moldic porosity (a pore formed by the dissolution of something which

you can recognize, • fracture porosity, • or any other type that may jump out at you.

If you do not know what to call it than apply the term pore. Most thin sections will have little porosity and what there is may be difficult to see. Some thin sections may have been impregnated with blue epoxy in which case the pore spaces will be readily visible as blue areas. Clay, when it occurs between grains is referred to as clay matrix. It may have been deposited as clay size fraction material during the depositional event or it may have formed as the result of a diagenetic process such as devitrification of volcanic glass; hard to tell in thin section how it formed. Note all the cements as pore filling material. The quartz outside of the dust rim on a detrital quartz grain is authigenic quartz cement. Calcite, gypsum, anhydrite, dolomite, and barite are all common cement minerals.

Don’t forget the unknowns and don’t forget to describe them (rule: length and detail of description is proportional to abundance). Hey, we all have unknowns; do your very best to figure out something but when everything fails call it an unknown. Well that takes care of space but also spend some time looking at the boundaries between the various items in the thin section. Look for and make note of concavo-convex grain to grain contacts (figure 2). microstyolite contacts (figure 2), etched surfaces, euhedral surfaces, all clues to diagenesis.

Figure 2: Types of grain-to-grain contacts that can result from pressure solution. Lots of stuff to seek out!!!! The big stuff pops out during your initial thin section survey while the rest will materialize during the modal analysis...keep a good set of notes along the way. So your first item of business is to survey the thin section and build your list of observed items for counting. List the framework grains first in your list since they had better be the most abundant items present. Then list the various materials that fill the space between the framework grains. Most abundant to least abundant. Below is a possible brief list.

1. monocrystalline quartz 2. polycrystalline quartz 3. feldspar 4. plagioclase 5. calcite bioclasts 6. glauconite grains 7. phosphorite grains 8. unknown grain A 9. blocky calcite cement 10. microcrystalline calcite matrix 11. intergranular pore

Part of the craft of doing this is planning ahead. You do not want to do a 300 count modal analysis and then at some other time do it again counting different items and you want your list organized to make it easy to use for several different purposes. Look at how the above list is organized. Items 1-4 are detrital grains listed such that the most abundant type monocrystalline quartz is first. Next follow all other types of detrital quartz grains. Then generic feldspar followed by specific feldspar types. These are the types of material that are used in many classifications of sandstone. Items 5-8 are accessory grains. Items 9-11 are what fill the space between the grains. Always leave open pore types last in the list. Now if I was to ask you what is the porosity of the sample? All you would have to do is look to the end of the list and add up the appropriate items (here just #11). If I were to ask what was the primary porosity of the sample then you quickly add up items 9,10,11. Note that, with the exception of the unknown, each has a mineral specified, that way you could quickly work out a bulk chemical composition. Or quickly determine if this were a limestone or a sandstone!

Sandstone Classification

What we will use here are classifications based on the results of modal analysis or in other words classifications based on the various components which make up the sandstone. A rule that I always go by and it would be a good practice for you to do also is that whenever you are using a particular classification you state somewhere just what classification it is that you are using and then stick to it strictly. This way one avoids some miss communication. Pick up any geology journal and look at the names in use and you will discover quickly that the same name is applied differently by different workers. In this lab exercise we will be working with the classifications of Robert Folk and Robert Dott. Each uses the term 'quartz arenite' yet that term means different things in each classification. If you were to describe a particular sandstone as being a quartz arenite there would be some question as to what you actually meant. But if you said quartz arenite in sensu Robert Dott, 1964, well someone could look up Dott 1964 and see just what you

meant. If you are writing a paper or report always somewhere in the Introduction or Methods state what nomenclature is being applied. Folks Classification of Sandstone Robert Folk’s classification of sandstone is based on the relative percentages of the major components of sands. The scheme uses a trilinear or ternary diagram (Figure 3).

Figure 3: Folk’s classification of Sandstone. Modified from Folk 1980. The main triangle or field has three poles:

• Q-pole: includes all types of detrital quartz grains (mono and poly crystalline) except chert.

• F-pole: includes all types of detrital feldspar grains (mono and poly) plus any granite and gneiss rock fragments.

• RF-pole: includes all other rock fragments: chert, limestone, basalt, slate, volcanic, etc. (except granite and gneiss)

To figure out where a particular sandstone would plot on this diagram go to the modal analysis. Add up the number of hits on the detrital quartz grains (Q-pole), add up the number of hits of feldspar, granite and gneiss rock fragments (F-pole), add up the number of hits on all the other rock fragments (RF-pole). Ignore everything else. Add these three groups of hits together and then divide each by that sum to get the percent for each pole. Q% = (Q-pole)/((Q-pole) + (F-pole) + (RF-pole)) F% = (F-pole)/((Q-pole) + (F-pole) + (RF-pole))

RF% = (RF-pole)/((Q-pole) + (F-pole) + (RF-pole)) Folk classification has seven divisions or clans (figure 3):

o Quartzarenite : all sandstone with 95% of greater quartz (Q-pole material)

o Subarkose: quartz ranging from 75% to 95% and the ratio of feldspar (F-pole) to rock fragments (RF-pole) being greater than 1.

o Sublitharenite: quartz 75% to 95% and the ratio of feldspar to rock fragments less than 1.

o Arkose: quartz less than 75% quartz feldspar to RF ratio greater than 3:1

o Lithic Arkose: quartz less than 75% quartz feldspar to RF ratio greater than 1:1 and less than 3:1

o Feldspathic litharenite: quartz less than 75% quartz feldspar to RF ratio greater than 1:3 but less than 1:1.

o Litharenite: quartz less than 75% quartz feldspar to RF ratio less than 1:3.

If you find the rock sample to be litharenite or a feldspathic litharenite and if you were able to identify the various rock fragments present then you could go further by subdividing these clans. Divide the rock fragments into volcanic rock fragments (VRF), metamorphic rock fragments (MRF) and sedimentary rock fragments (SRF) and determine the relative percentages of each like you did above. For example from a 200 count modal analysis 20 hits on SRF, 20 hits on MRF and 20 hits on VRF or 30% of the sample is rock fragments but of the rock fragments 33.3% are SRF, 33.3% are VRF, and 33.3% are MRF. Plot these on a trilinear diagram using VRF, SRF, and MRF as poles. This second trilinear diagram (figure 3) defines the three subclans of the litharenites: volcanic arenite, phillarenite, and sedarenite. You could go further with sub-subclan divisions again see Figure 3 for chert arenite, calcilithite, sandstone arenite and shale arenite. For the arkoses a similar game can be played using feldspar type (Figure 3). I suppose something could also be done with the various quartz types but I have never seen such.

Dott’s Classification of Sandstone

Dott’s classification, which is also widely used , differs some what from that of Folk. The Folk classification ignored the presence of matrix whereas Dott’s makes use of such. Dott like Folk uses trilinear diagrams but the definitions of

the poles differ slightly. Notably chert falls under the quartz or Q pole not under the rock fragment or RF pole. Also the divisions and names differ considerably. Look at your matrix percentage from modal analysis if it is less than 15% then the rock is one of the arenites. If it fall between 15% and 75% it is a wacke. If greater than 75% it is a mudstone. Note also from Figure 4 how the lithic arenites and lithic graywackes can be further subdivided like in Folk’s classification.

Figure 4: Dott’s Classification of Sandstone. Modified after Dott, 1964.